If you want to configure IMISH via iControl, this can be done via cli script.
How to use this snippet:
First, you need to enable the desired routing protocol via iControl REST. This can be accomplished by modifying the /mgmt/tm/net/route-domain/~Common~yourroutedomain object natively. Once enabled, you cannot speak directly to IMISH (as of 13.0 HF2), but you can create a cli script and pass variables to it, then write that to a temporary file and have IMISH suck it in.
You can call this script and pass arguments via iControl REST.