some time ago I had a mk2 astra gte 16v and that had factory central locking but no remote function
so at the time the MOS car alarms were all the rage and I got one and a locking module too
spent the weekend installing it and It worked a treat
I even fitted the same setup to a mk2 astra L that didn't have any central locking with the
help of locking servo motors from a volvo 240
so, yes it can be done and I'm sure there are alarm kits that you can do DIY or go to an ICE install
shop