ThomasV has stated multiple times that Electrum has and will always be BTC only.
As joniboni said, all the code is on Github... in fact, I'm even sure there are already DASH compatible "electrum clones". So, your best bet would be to engage some developers to modify that to work with your fork.
Refer:
https://electrum.dash.org/https://github.com/akhavr/electrum-dash