There are apps for use with asterisk app_rpt - for both directions, tx and rx.
The app the handles tx currently doesn't support NBFM, only p25.  The addition of NBFM tx is perhaps non trivial.
Nonetheless the asterisk-to-tx app is named repeater/src/python/usrp_tx.py.
At this time we are looking for experimenters interested in using the TX chain with hardware NBFM transmitters (no USRP required).  In this mode the user's FM transmitter is driven by a C4FM signal that has been software-generated in Gnu radio, and applied to the TX via a soundcard line-out port...
73
Max
      
  From: n4dav <n4dav@verizon.net>
 To:
 op25-dev@yahoogroups.com 
 Sent: Saturday, February 23, 2013 6:43 PM
 Subject: [op25-dev] Full asterisk support?
  
 
 
    
      
      
      Hi Folks, new here.. Did a lot of looking around, but didnt see the answer to this. I want to use app_rpt as my repeater controler and support P25 + NBFM. I see a rx-to-asterisk app here, but not a corresponding asterisk-to-tx app. I saw a repeater app but didnt appear to support asterisk. Am I missing something or does it not exist? Thanks!