The author would not like that and it's best to wait for 5.1, in which he might add back those textures.
However, it is perfectly cookable. If you just want a separate package that will be prioritized over HDRP 5.0, then simply place Skellige Rocks textures (XBM-only) you extracted from 4.6 into Uncooked folder, delete TGA files, cook.
If you want to have a fully-inclusive and merge-friendly cookable/uncookable HDRP 5 + HDRP 4.6 Skellige Rocks package (set of packages actually), then you're in for some manipulation because many textures will not be uncooked due to OUT OF MEMORY error that happens if a single texture.cache+bundle uses overly high-resolution textures that go over this game's set budget, but it does not happen if that package is isolated in a separate texture.cache+bundle.
Those uncooked textures will be tiny (2-3KB XBM) and have no TGA counter-parts. To get them, you must use Texture Parser/Viewer/Reader by Traderain (search for it), which will allow you to extract all textures in DDS format (filename.dds.dds), except for Pinus.dds, which it does extract, but files end up corrupted. You can extract all or just the ones that didn't get uncooked properly. You need to open those DDS files with something like Photoshop, Paint.NET, pr GIMP, and save as/export as TGA (32-bit, no compression) into correct directory, but in Modded folder (to import some as WorldDiffuse and those with "_n" at the end as NormalGloss), except for abies DDS files and creeper DDS files (NOT mesh hedera XBM/TGA though), which go into Uncooked folder. Do NOT include any pinus.dds files.
Hopefully you have the whole game uncooked because you need to make those tree DDS files uncookable and for that you should include vanilla SRT files that they come with when the game is uncooked and unpacked. HDRP doesn't come with them, which is why W3Oven can't uncook them, forcing the use of Texture Cache Parser/Viewer/Reader and the rest of related DDS files for that tree (don't over-write HDRP's tree DDS files though!). If you do not include the rest of DDS files for those modded tree files and SRT files (the way Halk did it), then DDS files will not be extracted upon uncooking and will not merge. They will be lost.
Then create 2 texture packages and 1 or 2 meshes package. I ASSUME you know to create 3 folders - Cooked, Modded, and Uncooked.
Package 1:
- Only include those gigantic brick texarray textures (WITH texarray file included) found in "environment\architecture\human\redania\nomans_land" because if you cook it with the rest of the textures, you will get OUT OF MEMORY error upon uncooking, which does not happen if a separate bundle with texture.cache is used
- I would not include any other files types, except XBM and texarray (no meshes, no mesh buffers, no w2p, w2l's, w2mg's, etc.)
Package 2:
- All other textures - in Uncooked (XBM-only and tree DDS-only) and those converted from DDS to TGA in Modded for import (with Skellige Rocks textures from HDRP 5.0 over-written with those from HDRP 4.6)
- No meshes and I advice against including water FX files for now, but you should include material FX files - w2l and SRT files (all of which go into Cooked folder)
Package 3 and possibly 4:
- All meshes and mesh buffers in UNPACKED format (using QuickBMS) and FX (unPACKED meshes will be much smaller in size/compressed compared to uncooked ones and that is fine)
- If you like water FX, then include water meshes and buffers and water-related FX also, but if you do not like them or undecided, then create package 4 with only water meshes and water FX only for testing
Took me 2 hours to do all that and I've been doing this for a while, yet still I couldn't get the pinus DDS files (only due to limited extraction tools...). It would be nice if someone could figure out why Texture Parser/Viewer/Reader does not extract them in non-corrupt format... Maybe overly high resolution? No idea...
Good luck - can't be any more helpful that this or it would require and epic amount of PM'ing and etc. Best to just cook Skellige Rocks textures from 4.6 and prioritize them over HDRP 5.0.