Hi Sascha,
You may want to take a quick look at my post to this discussion that shows how unit discovery can be used to create shorthand notation fairly painlessly . For example, you could use ctrl+= to define meters and centimeters or just do it by hand like so
m = Quantity[1, "Meters"];
cm = m /100;
18 m + 20 cm // N
(* Quantity[18.2, "Meters"] *)