How to, drag and drop an Assembly like a "library feature"?

For example, I have a stair assembly which consists of stair treads, stringers and hand rails.

We use Top-Down assembly methods, meaning the top assembly (master assembly) has a driving sketch of the major structure and includes some sketches of the Center Line of the required stair paths (from one level to another).

In my stair assembly, I've placed a part with only two sketches in it - zero features, just sketches. The first sketch is meant to control the length and angle of the stringer (the stringer is a separate part file in the stair assembly but is driven by this first sketch).

What I'm having trouble with is...

I want to be able to drag and drop the stair assembly into my master assembly and have it prompt for references like a "library feature" does and essentially make the two common stair lines equal length and coincident (essentially a "convert entities" type of sketch relationship).

Any ideas of how I can set this up?

Thanks,

Shaun