Yes, TPX does support the use of ALTER.
/* Below is a Sample to allow alter and system-managed rebuild
Note: it appears here as a comment, modify to your specifications
and uncomment. */
STRUCTURE NAME(XE16CFTPXAPPL) SIZE(20480)
We also provide messages to track begin and end:
TPXL5135 CF-EVT-EXIT: STRUCTURE ALTER REQUEST BEGIN, STRUCT yyyyyyy
Reason: An alter structure request has been initiated for the CA TPX structure yyyyyyy.
TPXL5136 CF-EVT-EXIT: STRUCTURE ALTER REQUEST ENDED, STRUCT yyyyyyy
Reason: An alter structure request for the CA TPX structure yyyyyyy has been completed.