Yes you can provided the application does not require functions not yet available using z390. With each new release of z390, larger and larger applications are being assembled and tested. An application written for TSO using TGET, TPUT full screen TN3270 plus QSAM, BSAM, or BDAM file IO should port with few or no changes. For a sample application see regression test program DEMOGUI6.MLC included with z390.
Assembler programs that are part of larger applications requiring other subsystems such as CICS or DB2 can be assembled and unit tested using test versions of interface macros.
If you have a small test case you would like to see converted and tested contact me.
z390 Assembler and Emulator