Yes, you can add up to 3 M5 repeaters to a base station.
When you add the M5s to the DECT chain you can select manual or local automatic. Choose local automatic so that the base can create the chain automatically. The first M5 should use the base RPN as its Sync Source. The 2nd M5 should use the RPN of the 1st M5 as its Sync Source, and the 3rd M5 should use the RPN of the 2nd M5 as its Sync Source.
Base/ RPN00 ----->> M5/RPN01------->>M5#2/RPN02-------->>M5#3/RPN03
^ ^ ^
| | |
sync source:RPN00 sync source:RPN01 sync source:RPN02
Note: The base has an omnidirectional antenna with a theoretical indoor spread pattern coverage area of 50 meters. Coverage is dependant on the physical structures over the covered area. In some environments use of M700 bases may be needed when coverage cannot be achieved using M5 repeaters.
ATTENTION: Please note that by using the first M5 the maximum number of usable M700s is reduced from 254 to 127. With 3 M5 there are even only 50 maximum bases.