Yes. Just change the maximum amount that can be accumulated to a number less than 1, such as 0.99.
Code:
recruit_pool "Men At Arms" 0 0.99 0.99 0 requires factions { england, } and hidden_resource i1 and hidden_resource i13
.
You'll never have a full unit but should be able to retrain.
My post here explains how to set up hidden resource indicators for each region. You'd need to insure all regions except the specified one have a pool like that above, and the specified one has a more normal pool. So let's assume you want "Tercio de Toledo" and have 199 regions, and you only want to recruit it in Toledo. Toledo is specified by the ID (i12, i21). You would need these pools:
Code:
;; Here's the pool for Toledo
recruit_pool "Tercio de Toledo" 3 0.5 5 0 requires factions { spain, } and hidden_resource i12 and hidden_resource i21
;; And these pools insure that all other regions have a different kind of pool
recruit_pool "Tercio de Toledo" 0 0.99 0.99 0 requires factions { spain, } and not hidden_resource i12
recruit_pool "Tercio de Toledo" 0 0.99 0.99 0 requires factions { spain, } and hidden_resource i12 and not hidden_resource i21
The first pool would only be true for Toledo. The second pool would be true for all pools except (i1-i12 i2-12 i3-i12 etc.), and the third one would be true for all those the second one misses, except Toledo. Make sense?