What you seem to be looking for is a Re/Set latch, most commonly the RS NOR latch. With a latch, someone stepping on a plate would trigger the latch to "Set", even after they step off the plate and stop sending that signal. The latch would then be reset by another input, like a button. One complication is that you would be unable to use levers with this method - RS NOR latches don't work if the Set or Reset inputs are constantly high (on). As soon as you would release the other input, the constant one would ultimately override it. Unless, of course, this is the functionality you want (for a lock, for instance). Check the Minecraft Wiki(s) for schematics on building an RS NOR latch (and learn a little about digital logic in the process).

In any case, what Chip says is true. This isn't WireMod; inputs cannot be forced to a certain state. In order for a lever to be switched, someone has to manually switch it. Pressure plates aren't really an exception, even though wooden plates can be triggered by pretty much anything (even a dropped item). If you're looking to hold a state when someone triggers an input, you're looking for a latch of some kind.