If you have them arranged correctly in your palette, you can drag-select 9 square tiles.
Or if they're not sorted correctly in your palette to do this, paint the 9 tiles into your scene somewhere as a square, as you've drawn above, and then alt-click-drag to pick up the square from your scene. It will then paint in edged mode.
There is an undocumented way to save and load brush presets which could help here. Open up tk2dTileMapEditor.cs, and insert the following first line in the file:
#define TK2D_TILEMAP_EXPERIMENTAL
This should be the very first line before anything else. Once that is done, you'll get "Load" and "Save" buttons - you should be able to load and save brush presets now.