Please check package MLFB  e.g. 6FC56 00-0AX0X-0AA0. Is it miling or turning?

Confirm the additional axis is rotary or linear.

MrPiNG

Milling

How do I check MFLB ?

MLFB number 6FC5610-0BA10-0AA1 which you mentioned is of PCU for 4 axis + 1 spindle. It is OK.

You need to check & confirm s/w installed MLFB number, i.e. it will confirm possibility of addition of 4 th axis. (operational point of view)

Have you worked out requirement of aditional I/O to interface with system? Check if those can be acomodate with existing PP modules. If not, then you need to add additional PP module.

Please check the power capacity of existing E/R (power infeed module). Work out if it is sufficient to supply power to LT module of addional axis motor. You want to connect additional axis LT power module, motor encoder & need to re-configure system. 

HTH. 

Yes, you can use but since you can add only 1, going for double module will be wastage of money..

In order to add 1 additional drive + motor, you need to configure its address, telegram type, encoder connections, motor data etc. etc. & need to tune additional axis.

Please referSINUMERIK 802D: Turning, Milling diagnosis manual &802D: Turning, Milling commissioning manual &SINUMERIK 802D: Milling programing manual for more detail information.

HTH