Customizing picker files/grep etc. by appending commands

[idelice]

I wasn't sure how to formulate the title to be understandable but what I'm trying to accomplish is the following:

1. Open snack grep
2. Type "foo"
3. Trigger custom command e.g. <c-i>
4. "foo" now becomes "foo -- -t=lua"

Is this possible and if so, how?

[drowning-cat]

#1701 (comment)