Can I add special characters to the default Android keyboard? Sometimes I need to type a special character or symbol that is not accessible by default through the Android keyboard. Currently I do this by copy/pasting them from other sources, very inconvenient.

Is there a way to add custom characters to the keyboard? 

It's typically things like currency symbols (such as ₭ ₩ ฿ ƒ) or math symbols (such as ∈ ⇔ ∫ ⌊ ⌋ ∑ ) etc.
No, you cannot add custom keys on the default Android Keyboard.

But what might work is keyboard shortcuts. You can create shortcuts like when you type "yt" option to change to "You there" will appear. So may be you can create keyboard shortcut that when you type "ms1" it will change to ₭.

To create keyboard shortcut:


Go to  Settings -> Language &amp; Input ->Personal dictionary
Tap the “+” sign in the top-right corner of the screen and create your shortcut.
Now, go to the Messaging app, compose a new message, and tap one of your new shortcuts—and when you do, the phrase you entered will appear in the row of auto-correct options just above the keyboard. Tap the phrase to add it to your message.


More details on keyboard shortcut is here: http://heresthethingblog.com/2012/12/04/androidiphone-tip-create-custom/