I recently replaced my Apple Airport Extreme and Express with a RB2011UiAS-2HnD-IN however they WIFI signal strengh in my garden is still to low. Can I use one of my Airports as WDS AP point conected to my Mikrotik or should I buy an additional router like the RB951Ui-2HnD to extend my wireless network?

Hi,

Well you can always use Airport extreme in WDS mode to extend your network. It can be done without any problems.
Check documents:
http://support.apple.com/kb/ht4262

There is maybe issue with network speed, so be sure that MKT and extreme are on same wifi speed ( G or N standard).

I hope that this helped ...

BR. M.