Possible? Very much so: Video of prototype
The thing is that you need to know quite you're way around coding in events to do so. But unless you'd just use a pre made script, you'd still need the same knowledge of the concepts to make a script out of it.
The biggest difficulty with creating a TBS is making a reasonable enemy AI. For it to be reasonable, it needs to be able to find it's way around obstacles (both from the static terrain, and the dynamically positioned enemies), and you might want different terrains to cost more to travel (like woods in advance wars). And there's the issue of who the enemy should choose to attack, how to position itself if it's a long range unit, how to position the attack itself if it's an aoe attack and so on.
To give you a better idea at the complexity of making a "reasonable" TBS in rm2k3 you could take a look at these 2 tutorials:TBS part 1TBS part 2
And there are still two more parts to come before the tutorial series is complete and can construct a full TBS.
If those tutorials look like a bit too much or hard to grasp, but you still want to learn how to do it in rm2k3, then you should probably try coding something simpler first, like a custom menu system.