Sadly, I find that I must disagree with you. To me Sentinel is proof positive that Uniden is a hardware company and should sub out it's software development.
I can use Butel or ProScan and get a better set of Favorites Lists, Systems, Departments, Talk Groups and conventional frequencies in 1/2 the time it would take me to do it in Sentinel.
"Steep" is a nice way of saying over convoluted and complex programming choices.
The last time I saw software this frustrating to use was Motorola MTSX, which was DOS based.
All I use Sentinel for is changing settings and updating firmware. I also update the RRDB information, but only use that if I'm driving through an area and don't have the time to "roll my own" FL and so on.
As to the original question, it can be done, but it's not easy because the memory structure doesn't really support just programming in a conventional frequency unless there is an existing System and Department in which to drop it.
It might be easier to just program it in manually, but there still has to be a FL, System, and Department to put it into.
I think you can still get a free 30 day license to try out Pro Scan. The learning curve is not particularly steep and after a trial you may well decide it's worth the price to buy the license.
"This crazy Sentinel thing"☺️ is the premier programming tool people have been using for the 9 years the x36 scanners have been available. It also programs the newer SDS 100 and 200. ProScan is excellent also.
It gives you access to the database and is a way to update your firmware which is the first thing you should do when you receive your radio right after you enable all of your service types that you can modify later.
I'm wondering if you have updated your firmware and when the last time was as many improvements have been made on the 436.
Nobody will deny it's a pretty steep learning curve but the documentation sent to you here should really help and reading what's available on radio reference. If you search here you will find so much that will unlock a new world to you. If you get hung up just Google it and it'll take you straight to threads in the forums of radio reference that refer to your question..
First thing you want to do is update your database and firmware.