One option is making/using a Custom Action. As example:
Code:
Script: Thonex_Set time selection to region at edit cursor.lua
Insert new MIDI item...
The first Action is available via ReaPack.
Another possible (two-step) option can look like this:
1) Setting Preferences > Mouse Modifiers for [ Ruler ] [ double click ] ** to
Set time selection to region - so you can make a time selection via double click on the region.
2) Using 'Insert new MIDI item...' (or the assigned shortcut) afterwards to insert an Item into the time selection
** you might have to check the Mouse Modifier for [ Ruler ] [ left click ] as well in this case since some left click assignments might be interfering with the double click one
EDIT: I just saw that I overlooked the 'selected' (region) part in your post. In this case using just the 'Insert new MIDI item' Action (from the Insert menu or the Action List, etc.), as Greg also mentioned below, should be enough.