Can I use an outboard Shimano BB on a Trek madone ALR (alum.) with pressfit BB?
No. The BB90 design puts the bearings the same distance apart that a normal external Shimano BB would, hence a normal Hollowtech 2 road crank being able to run on it.

It may be possible to make a BB386EVO adapter for BB90, but if so I don't think anyone's done it.