@girish SnappyMail should support the rainloop script as well (it's in the list named "rainloop.user").
Difference is that SnappyMail also supports all other scripts and you can creat a script to include multiple scripts.
I made it this way because i also use Kmail (and others like Thunderbird) and then RainLoop conflicts with them because in Sieve you can only enable 1 script and RainLoop disables them in favour of rainloop.user
So you could finegrane your instructions how to combine/include scripts and still use the gui on the rainloop.user script.