You are correct. Adding an external axis results in 6+1=7 DOF.
Caveats...the same as for any redundant system
- solving IK requires a more complex approach (and then path/trajectory planning also)
- precision/repeatability/accuracy can be affected if the external axis has a worse precision/repeatability//accuracy
- same goes for dynamic performance
- technical compatibility issues might arise (depending on how the external axis is controlled and what communication protocols are available at the robot controller)