Inokatsu m4, HPA?
 

I have an inokatsu M4 Is it possible to purchase a drum mag that works with an HPA line, or is there a kit? Or is it strictly a personal modification? There is this company alyanairsoft.com, but it's based in the UK. But I believe it's for WE only.

Yes it can be done.
The Japanese have DIY'd it. The HPA mounting kit can be found from RA-tech I believe and you would need to customize and gut the drum mag.