You could try connecting the DRO encoders to a DC servo drive but unless you had near-zero backlash, the drive could be almost impossible to tune. This also assumes the encoder signal is a compatible (TTL A/B quadrature) type. If you have no backlash, it might be a workable solution.

Do the Indramat motors have encoders built in already? If not and the linear encoder doesn't work out, you'd need to fit a rotary encoder to each of the motors so the CNCdrives could work with them. You can buy suitable Haidenhain clone encoders from China for peanuts. Failing that, you'd probably be looking at acquiring some proper AC servos which would cost a lot more and need mechanical buggerage to fit them where the Indramats were. Not sure I'd want to waste my time and money fitting steppers to a decent machine like this.

Be aware that early analogue DC brushed systems often had tachometers mounted in the motors. They can't be used as encoders, as they only report speed, not position. However, you could remove them and use the freed up space to mount encoders.