add LTM in ha pair to GTM

I assume that you are wanting the LTMs to do monitoring on behalf of the GTM ...

Without regards to the fact that they are combined into an Active/Standby HA Pair ... can you define the two separately, each one with its own non-floating self-IP ... then define the two of them as a Prober Pool -- choosing to load balance between the two LTMs via the Global Availability load balancing method.

I would think that this would allow the one that is active to always be counted on to respond correctly to monitor probing ...

Please report back as to whether this works ... or if you found a way that does ...