Use all units in campaign

What you could possibly do is create a custom campaign using the map files from Wings of Liberty/ Heart of the Swarm, and tweak them to give yourself the units that you want.

• Open up the SC2 Map Editor, and select the 'Open Folder' menu
• Click on the campaign tab. A warning should appear about spoilers. Accept it.
• You should see a list of files. These are the actual in-game mission files. The Heart of the Swarm maps should be in a folder called swarm.
• Open the maps, and save a copy of all of them (in case you stuff up and need to revert back.)

Once you're done, you could access the maps from your custom campaign directory in game.

Here's a more in depth explanation of how to find the campaign mission files