I'm working on a mod that does almost exactly what you want, and I intend to release it just as soon as patch 1.1 goes live.
How the mod works:
Pioneers require a resource called "Settlers".
This resource is acquired two different ways.
1. You receive "settlers" when you level up a city. (enough for one pioneer per level of the city)
2. You receive "settlers" by reaching certain fame milestones. (enough for one pioneer per fame milestone)
What is left to do on the mod:
1. Finish adjusting the fame milestones for settlers
2. Double check and correct an issue where the resources given and used are doubled when the mod is installed exclusively to the mod folder (I think this issue has more to do with me fiddling with the core files rather than an issue with the mod.)
3. Adjust the size of the settler resource graphic. I'm using the settler wagon from Master of Magic, and it is a tad too small.