I have some experience with editing the pak0 file (having created my own) but i've never came across not being able to import the model. Ive had it freeze during saving the edited PAK, but never has importing it been a problem.
When you say you cant import them, do you mean when you go to "import files" and select the said files, that they do not arrive in the list of the PAKs contents
It may also be an idea to place all the edited/new models in its own folder and importing this instead of individual folders. That is what i always done...