You can use a pre-stain wood conditioner on appropriate exterior wood without any problems. A conditioner is used on softwoods like pine or fir. It isn't used on hardwoods.
I can block the absorbency of stain if too much is applied or it is allowed to fully dry before applying the stain. Make sure to read the manufacture application instructions carefully as there are differences between manufactures.
If you have a door made from softwood then using a pre-stain conditioner is a good idea. Especially if the stain color is dark. Very light stains don't give the same volume of problems.