Is the door surface embossed, has a wood grain texture? If it does everything will be easier. If not you will need to add the grain using some graining tools and techniques.
Door surface embossed; this type of surface is fairly easy to stain but the door will need a neutral (light-medium tan) base coat or primer applied before staining. Staining is the same as with any embossed fiberglass door. Check out <a href="https://www.house-painting-info.com/staining-a-fiberglass-door.html">Staining a Fiberglass Door</a> for more information.
Door surface smooth; this will be difficult to do. This surface will require the same base coat as above but the stain will need application using wood graining tools. Check out <a href="https://www.house-painting-info.com/oak-wood-graining.html"Oak Wood Graining Using Stain</a> and <a href="https://www.house-painting-info.com/wood-graining-tools.html">Types of Wood Graining Tools</a> for more information.