I did quite enjoy cardamom flavoured coffee. Currently I use aeropress to brew coffee at home. Is there any way to add cardamom to the brewing routine?
You'll probably be able to get away simply with ground cardamom/cardamon added to the grounds when you're "pre-infusing"/steeping the coffee grounds in the AeroPress. It's probably best to use very finely ground, freshly ground cardamom seeds. You may even wish to grind the cardamom seeds along with your coffee (i.e., in the grinder at the same time); you'll get similarly-fine grind. However, you might get a bit of cardamom flavour left over in the grinder; this may (not) be desirable.
As another option, you could also consider a sprinkle of the dry, ground spice into the cup after brewing.
In addition to cardamom or black cardamom, one might also consider cinnamon, nutmeg, cloves, allspice, anise, or other "baking spices." Any of these could be used in a similar way. For example, I sometimes use freshly grated nutmeg in the grounds when brewing pour-over, and it gives a nice smell and a hint of flavour in the cup.
You might also consider using an extract or essential oil or some high-intensity flavouring like that. You'd probably want to add that to your cup after brewing.
I see a few problems that you might have to work around, specific to AeroPress, that you might run into with any of these spices. One problem you might encounter is the rather short infusion time commonly used in AeroPress; an option might be to try to steep the spice in the water as you're bringing it to the boil, but you risk burning the spice or otherwise contaminating the kettle (but might be worth a try to boil in a regular cooking pan). Another problem is that some spices have a rather strong hydrophobic tendency (specifically, cinnamon), so it might be necessary to stir the grounds more thoroughly than usual, to make sure the spices are infusing in the water. Also, these spices might leave behind some flavour in the AeroPress, so do consider washing thoroughly and soon after the brew. And you might get a hint of the spice next brew... so make sure it's something that you won't mind in the future!
EDIT: More thoughts based on a later question: There's a few other suggestions from recipes like this and this (somewhat arbitrary top search hits). The former suggests using a French press / cafetière (providing longer steeping time for the spice) and the latter recommends grinding the spice with the coffee beans.