Can you add hiking trails without knowledge/permission of landowner?

I would add the paths and use the appropriate access tag.

First off welcome to Openstreetmap!

There is no obligation to get permission from the land owner to map the trails on their land, but it's always good to be clear what you are mapping. Mapping a trail that is inaccessible to the public, and omitting the fact that it's inaccessible is unhelpful and misleading.

Adding the access tag is valuable to make the map more useful for its users, as is adding any barriers to access. So adding a node tagged as a gate in the location of a gate blocking the trail is really useful. If you come across any signs telling you to stay off, or people telling you to stay off, adding access=private to the trail is a good idea. If you suspect that the land is private, and there are no rights of access for the public, but don't know it, you can add the access=unknown tag to the path.

Of course always follow local laws. I can't help you out there, as I know nothing about Thai laws.

Hi Tinnic, it’s always preferable to start asking the owner if a trail or route is allowed over the plantation or any other farmland. In more populated areas, people or firms don’t like strangers on their grounds, just ask it won’t harm. But a trespasser could be harmed if the route is mapped or signed over a guarded area.

Quick comment to add onto these great answers...

OSM should reflect what is visible at that location. If there is a significant trail, then it should also be shown in OSM. Record all those navigational landmarks.

The conditions of the trail (including land use permissions and typical usage) have secondary importance on the map.

Happy Mapping 8)