I'm not aware of any standard REXX, TSO, or ISPF commands or services that will concatenate datasets. That process is usually handled by the system, either by the way the datasets are allocated in the JCL, or in TSO by using the TSO ALLOCATE
command. In ISPF, you can use the QBASELIB
Service to obtain the current Library information for a specified DDNAME. You can combine these two together to add datasets to an existing DDNAME.
In TSO/E, you can use the ALTLIB
command to define alternative application libraries for REXX execs and CLISTs, if that is your real intent with this.