One of the best things about these Passport systems is that the speakers are well matched to the amps. That's a big piece of what makes the sound so clear, and makes the system reliable. Add more speakers, and you no longer have a match between the speakers and the amp. You could burn out the amp, and even if you don't, you won't get "more sound" out of more speakers if they are all driven by the same amp. The amp has the wattage that pushes the speaker cones in and out to make the sound. The volume comes from the amount of air pushed (which is linked to the wattage), not to the number of speakers.
You could connect the "Stereo Out" port on the Passport 150 into "active" speakers (speakers that have an amp built into them), or patch it into another amp. I do that with a second Passport. That gives me four speakers AND a second amp to push them. I have a Passport 300 Pro and a Passport 500 Pro, so together, I have four speakers pushed by 800 watts.
The important thing is that the extra speakers or amps need to accept "line level input" because the "Stereo Out" port is a "line level" port. The signal is not strong enough to run speakers without another amp.
If you feel like it's not worth the money to get another amp or "powered" or "active" speakers, then it's probably not worth trying to upgrade your system. It's better to not do it, than to not do it right. Hooking more unpowered/passive speakers up to your Passport 150 is not doing it right.