There are several ways of making more lilac's..
Probably the easiest, is to take a sucker from below the plant.. *it would travel under ground and come out a bit from the momma shrub..
~could be trick to find one, if you lawn mow or weed whack right up to the base..*
Shovel a spade thru the dirt, under the sucker, and replant or repot..
It can take a bit to reflower though..
*I have two done that way and they are gorgeous, but had to wait a bunnnnnnnnnnch of years for them to flower*
or you can get yourself a nifty gizmo like this...
Lee Valley Tools, Propagation Pots You can clip those right on to the bush now and do it..
*or you can be ingenuitous and put some peat moss, in a halfed zip lock baggie, cut off some of leaves, half cut thru the limb, and maybe slip in a bit of root tone *a rooting hormone*, cover the baggie over the wound and ductape it closed, wait a few weeks and it should work...*
You can try layering, where you take a branch, and strip a few of the leaves and then put it down on the ground, *still attached to momma plant* and put something heavy on top of that branch for two reasons', to hold it down and to give it good contact to the ground for it to root...