The easy way: find an Arabic keyboard on the Play store, and switch to it when you want to type in some Arabic text.
The hard way: what I did.
To type in this tweet (a quotation from @GWillowWilson's Alif The Unseen), I wrote the English text first. Then, I switched the tablet's system language to Arabic (changing it for all users) to type in three Arabic letters, and then changed the language back to English. Annoying.
If I'd installed an Arabic keyboard, I could've just selected the Arabic input method from the nagivation pull-down. It's weird that the system lets me switch between a few languages without changing the system language, but not Arabic (which it clearly supports and has fonts for).