If the application you’re using to create your MIDI files supports MusicXML export, you might try exporting MusicXML rather than MIDI in order to import the data into Dorico.
The way to do the edit you want to do is using the following steps, but it’s far from fast:
- Select the quarter triplet bracket and delete it, which unscales the notes to be regular non-triplet quarter notes, eating up the quarter rest on the second beat.
- Select the second quarter note and change its duration to an eighth note.
- Select the first quarter note and type Shift+N or Return to show the caret. (You have to do this, or the next step won’t work.)
- Type 5 to change the duration to eighth; this is specifying the unit of the tuplet you’re about to create.
- Type ; for the tuplet popover, then type 3:2 into the popover, and hit Return.
However, I think you’re going to find that it’s quicker to fix this once, then copy and paste the first two beats of the bar into other instances of the same pattern, repitching the pasted notes according to the ones you replaced.