The input needs to be an on-off range for the IF+ statement to work, sure.
You can use a UI or a high speed input, either will work fine with the IF+ statement.
I am still not clear what you mean about the value and the accumulator but in short, you configure the input as ON-OFF type. Then you use a small program to count them and add the total in a VAR.
10 IF+ IN1 THEN VAR1 = VAR1 + 1.
To read the value of var1, you just read it like any normal analog variable… or if you are reading from an external system you can read VAR1 as a bacnet AV or modbus register.
Maybe I am missing something in what you are trying to do but at first glance it seems like a pretty straightforward application to count pulses on one of the inputs. Here’s a link to the specs of the inputs and how long the pulse needs to be, its 0.01 second for the high speed inputs and 0.5 seconds on the regular UIs.