HTBASIC cannot load native HPBASIC PROG Source. Even though you were able to convert the DISK FORMAT from LIF to DOS, the PROG File contents are still in HPBASIC P-TOKEN Format. The only way to do what you want is to obtain a HPBASIC Workstation, LOAD the PROG File into the BASIC Editor and then SAVE the program, which will save the program lines as ascii text.
HTBASIC can then GET (rather than LOAD) the program file. Execution of the file will probably require some fix-up to port the code to HTBASIC.