Hey Pariah,
Sure you can – here's an example:
Keyboard Maestro “Paste Dated Attribution Action v1.0” Macro
It's unlikely Peter will ever add this functionality – because demand is not all that high, and it'd be complicated to do.
However – our intrepid and talented friend @DanThomas who was a software engineer in another life built KMFAM for this very purpose.
KMFAM is an outstanding piece of work – is quite comprehensive – is free – and is pretty turnkey.
Only a few folks have had much trouble installing it, and I believe Dan has been able to help all of them overcome their issues.
I would recommend this option, before you even consider trying to roll-your-own.
If you do decide to build your own mousetrap then you can use my macro above as an example.
To create your own custom actions you have to manually create the action with all your custom attributes and then copy it as XML.
My macro shows how to use that XML to create a new action – although its not completely straightforward, since it's part of a purpose-built macro.
If there's more interest I'll post a more basic example.
You can also create template macros in Keyboard Maestro with one template action each and use a Prompt With List action to insert them into the working macro.
I'm working on a proof-of-concept for this right now, but it's poor man's fare compared to KMFAM.
-Chris