Well, it all comes from wanting that all pioneers could build roads, not only the mancers.
I have no idea of modding, and I have not seen a "guide for FE newbie modders yet", but Heavenfall showed me a code that seems to add the trait "roadbuilder" (https://forums.elementalgame.com/434562/page/3/#3284806)
And it worked: now all my faction pioneers got the roadbuilding trait. Here is the xml
http://dl.dropbox.com/u/24075181/RoadbuilderPioneer.rar
EDIT:
Added a trainable Roadbuilder Unit for all factions. It has an upkeep of 0.9 gildar per turn, and 75 production
To install, just drop the xml file in /Mods folders and be sure the "Use Mods" option is on
http://dl.dropbox.com/u/24075181/RoadbuilderUnit.rar
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But when testing, more things came to my mind. I hope you can help me with this 
1- I've noticed, but not sure, that the pioneers now take more to build. Is it related to this?
<Cost>10</Cost>
2- I though in creating a roadbuilder unit, instead, by using all the info of the pioneers, calling it
<UnitType InternalName="Unit_Roadbuilder_Altarian">
And deleting what seems to settle/create outpost in <GameModifier>
All the rest was the same. But no new unit was available. What am I doing wrong to create that kind of unit?
3- I wanted to give an upkeep to that unit of o,3 gildar per member. But first, I need to create it.
Anyway, I've seen <HasWages>0</HasWages>
Is this related to it too? Should this go with 1?
And then add <WageOverride>0.3</WageOverride>?
4- Parrottmath suggested a cost per road they build. Like 2 gildar per road piece. I would like to know if I can do that too. If I can't easily (for my scarce knowledge) create a roadbuilder unit, I find OK that each road piece costs 2 gildar. Maybe 5.
Thank you 