Is there any way, given a function passed as a parameter, to alter its input parameter string before evaluating it?

Here's pseudo-code for what I'm hoping to achieve:

test.func <- function(a, b) {
    # here I want to alter the b expression before evaluating it:
    b(..., val1=a)
}

Given the function call passed to b, I want to add in a as another parameter without needing to always specify ... in the b call. So the output from this test.func call should be:

test.func(a="a", b=paste(1, 2))
"1"  "2"  "a"