Bryce can do some serious models.

If you want to use other software to make your models, then you just need to make sure that the model you want to put into Bryce is in some format Bryce likes. There are standard formats out there nearly any good model making software can export to: obj, 3ds, dxf, lwo.

This forum has many threads concerning free software to do models. For example, there's at least two good threads for the Ivy Generator, which exports obj models. There is also the free CB Modeler Pro which does well for organic models.

But Bryce can do quite a bit.