I guess, if your import worked correctly and your modifier button does not highlight, then the original author has forbidden modification. Possible solutions: search within this platform whether you find a site containing the code; ask the author for a modifiable version; (no, I am not teaching file hacking here 😉 ).
Not to my knowledge, but others might know more than me. Although I am a fan of open source, please keep in mind that people who restrict their code have put a lot of time in it and maybe that’s their earnings, so it is legitimate.
The last solution I could think of is: spend your time and sweat into writing your own code. Sorry but that’s all I can suggest.