Can I compose a function in VBA and refer to it in a
formula?

Let's say A1 has the text: "aja,ajsd," and I want to use a
formula to call the function I've created:
=ParseComma(A1)
or perhaps qualifying it more? =Module1.ParseComma(A1)
to return "ajaajsd" to the field?

If I can't, why not? Can Add-ins do it?

Thanks for the info.

-Brad

PS: I'm not interested in a solution to parse a comma, but
thanks. :)