More radius value on attributes?

Is there any way adding more than one attribute value of the same “operator” on MT generated attributes (Vendor 14988)?

my idea:

This does not work

username attribute op value
----------------------------------------------
operator Mikrtotik-Address-List = addresslist1
operator Mikrtotik-Address-List += addresslist2
operator Mikrtotik-Address-List += addresslist3

as it is for example on attribute Framed-Route

This Works

username attribute op value
----------------------------------------------
operator Framed-Route = "10.200.100.0/24 10.100.100.1 88 2 -1 3 400"
operator Framed-Route += "10.200.101.0/24 10.100.100.1 88 2 -1 3 400"
operator Framed-Route += "10.200.105.0/24 10.100.100.1 88 2 -1 3 400"