It's been a while since I've used 0.18um but there should be a separate model most likely named nmos_hvt or something, I even forgot the naming convention unfortunately. But in the process documents there is actually a list of available devices which is exactly what you should read. It will also direct you to the correct models you should use for the device you want to use high Vt device in this case.

If you mean to ask how they're implemented in layout, there's actually a layer that specifies the Vt of devices as far as I remember. Again refer to device truth table for the name of the layer.

If you're asking how the Vt is adjusted, that's not my expertise but as far as I know ion implantation is the most commonly used method, you can read more about it online.

Thank you for your response guys. I highly appreciate it.

Are you getting this error while simulating your circuit? if so, add the mos.scs to the ADE model library.