Kia Ora

I'm having a problem with old practice's.

1/ make a copy of a doorway nif and rename it
2/ use nifskope to retexture it as I need and save

this is where it turns to custard
3/ try to drag and drop it into CK as a static item
or
duplicate the item in CK that I'm retextureing and rename it
go to base item and try to select a new model, this I can't seem to do

Has anybody gotten this to work yet please?

Thanks in advance

Edit:
To be clear this is not a rip or a newly made model, it's a wal from in game I want to retexture to match
others from a different set