Looking for an (alsa) input plugin

Just started after some quite period to get my Mopidy running using Stretch on a RPI2.
I want to ‘connect’ my USB sound input to my AMP2 simular as connecting a (radio) stream. and using the webclient volumn control (which uses alsamixer)
Searching the net showed me musicpd.org ‘input plugins’. I understood these are not usable in Mopidy.
Can some tell me how I should do this? Or point me to additional sources.
Or should I create a input-plugin my self?

Best regards,