Welcome to my world.
You are correct, this is not possible. ACI wants you to use transit networks for transit and stub networks as stubs. Period. What this means is that you will need to either make VLAN 100 an L3Out as opposed a tenant BD, or you will need to find a new home for the next-hop toward 10.10.10.0/24.
In our case, we did a little of both.
Scenario 1 - Migrate VLAN 100 to an L3Out
We had a particular subnet that was used entirely for the purposes of virtual server addresses on hardware load balancers. And it was also used as our transit network to subnets "behind" that load balancer. So we had something like this pre-ACI:
ip route 10.1.1.0/24 10.1.100.1
ip route 10.1.2.0/24 10.1.100.1
ip route 10.1.3.0/24 10.1.100.1
interface Vlan100
ip address 10.1.100.254/24
!
What we did in the above case was create an L3Out called HLB-Transit_L3Out. It was deployed to our border leaves as an L3Out SVI. The SVI was configured with a secondary address of 10.1.100.254/24 on both sides. We were then able to create the static routes on our border leaves without a problem. We then redistributed those into our fabric L3Out to advertise these subnets to the rest of the world.
Scenario 2 - Migrate router out of VLAN 200
We also had another case where we had VLAN 200 which was a typical data center server farm. Shame on us, I know, but we had static routes with a next-hop of an router that was in this same VLAN 200. So it looked like this pre-ACI:
ip route 10.1.201.0/24 10.1.200.1
ip route 10.1.202.0/24 10.1.200.1
ip route 10.1.203.0/24 10.1.200.1
interface Vlan200
ip address 10.1.200.254/24
!
Because this was a server VLAN, we had no choice but to re-IP the router and create an L3Out. So we created an L3Out for this router, created the static routes on the border leaves, etc etc etc. But in this case, it was a new subnet for the L3Out: 10.0.0.200/31. We were then able to move our VLAN 200 into ACI as a regular tenant BD.
