It is already there - I think you are trying to make a connection from the assembly to the sub-assembly. For that you need an EXECUTE statement in your main assembly program to pass down any parameter mappings/values that you want.
There are a few other methods that work for this, assembly relations are an example, but you must be sure to avoide circular relationships.
Always TOP-->DOWN. So you must add the execute statement in the Assembly just above the part or sub-assembly that you are trying to send changes to. You must also make sure that your parameters are mapped properly for their type so that the values do not create a regen error.
Hope this helps.
p.s. you might want to check in to p-Program - it is a graphical Pro/PROGRAM editor and has all of these rules built into it - also wizards to handle these not-so-common functions.