You certainly can.
Q. Can I transfer more than one old ISA into a new one?
A. Consolidating all your old ISAs into one is allowed, and often a good way of upping the rate on your whole wodge of ISA cash - this is because providers regularly slice rates on the money you saved in previous years.
To do this you just tell the new provider that you want to transfer in from multiple old ISAs. The main thing to watch here is that it doesn't push your combined savings with one financial institution over £75,000, as then you'd no longer be within the UK's savings safety limit (read Are your savings safe?).