Can I add a AP(non Orbi) to my current Orbi RBR50 system?

The answer is "Yes" (and "Not completely")

Any WiFi access point can be connected to the Orbi and will function as if it were connected to any other WiFi router.  Devices can connect to it and the Orbi (or base router if the Orbi is in AP mode) will assign it an IP address.  It can reach the internet and any other device on the local network (LAN).

EXCEPT... this new access point will NOT be "integrated" with the Orbi network.  The Orbi system will not think of it as a "satellite".  Devices will not "roam" between it and the Orbi units (and back).  If it is on the same WiFi channel with the Orbi units, it will compete with them.  If Orbi decides to change WiFi channels, it will not change with the Orbi units.

Generally, adding a noni-Orbi access point makes sense if there are devices which cannot connect to the Orbi WiFi and do not move. (Such as: garage door openers, security cameras, sprinkler timers, door locks, power plugs, etc.)

This access point can have the same SSID/password as the Orbi (for simplicity), but generally should be on different WiFi channels than the Orbi.  And, the user should be aware that "roaming" will be complicated.  Basically, a smartphone will have to "give up" on the Orbi network completely, look around for a new network, and say, "Hey.  Here's one with the same name.  Cool."  Just like if there were WiFi systems in two buildings across town with the same SSID/password.  The smartphone doesn't know which one it is, only that it knows the SSID/password and can connect.

Sorry to rant on.