I've never tried but apparently it is possible to root lilac cuttings. Advise I've seen online usually recommends using rooting poweder/gel and planting into pots of compost mixed with sand. You need to take a softwood cutting, i.e. cut the twig where it is not too old and hard but more bendy but not just very soft new growth.
@dan5584 my lilacs usually have extra ones growing round the bottom, if your friendly with your neighbours ask them to dig you one out if theirs does the same. Usually not the best time when things are in flower though, probably better in early spring or autumn 😊
Dan, take a 3-4" cutting below a leaf node and place the stem in a glass of water. Take off the leaves from the part of the stem that will be in the water to keep the cutting from rotting. Change the water weekly to keep it fresh. Should start to root after a few weeks. Keep in partial sunlight.