Replacing 18650 batteries for AA batteries
Data point: 2 x 18650s will go from about 8.4V to 6V during discharge.
4 x AA Alkaline will go from about 8.2V to 5V during discharge, so voltage range is similar. 

A top AA Alkaline has ABOUT 4 Wh energy - depending on discharge rate. Some cells have full data sheets available.
A 3 Ah 18650 LiIon has about 11 Wh energy.

So 2 x 18650 LiIon = 22+ Wh and 5 x AA ~~= 20 Wh - so they are reasonably comparable. 

However, as others have noted, current capability from AA Alkalines will usually be lower. 

If you can manage to use 2 x 18650s it will be 'more satisfactory'.
There is a very small chance that the Robot. uses a split supply (+/- battery relative to ground) - in which case you cannot use an odd number of replacement cells. [As they specified Vbat needing to be >- 7.2V the split battery idea is unlikely]