Can you buy the all terrain/jogging tires for the Britax B-Agile?
Officially, no, I am sorry to say the B-Agile can't really be all-terrain. I mean, there were these optional fixed wheels (an axle with non-swivel wheels accessory) available, but it is almost impossible to get now, AND it didn't make the pushchair much all-terrain, just a little less hard to get over bumps - but ot was still very rattly, not much suspension there (the wheels were foam-covered only).
However, there is a thing moms do with it, and that is getting the spare set of Britax B-Motion wheels that are inflatable and putting it on the B-Agile buggy. YOu need to keep in mind that the rear B-Motion wheels will not fit the best in terms of brake-functioning - to keep the brake working, you need to change the axle (silver pin) in the rear wheels for the pin from the B-Agile wheels. Also, be ready that it takes a bit of time to get it right, putting them on the B-Agile frame, but it is possible.
Please, bear in mind that the B-Agile is an urban model and an urban chassis, so it will never be springy and soft in rough conditions. I hope I helped, this is all I can think of in terms of making your pushchair more all-terrain.