How can I send an iMessage from my iPad to my iPhone?
Yes. Sending a message to "yourself" (i.e. your other devices) is done exactly how you would send a message to anyone else. Start a new conversation in Messages, then enter your phone number or email address, whatever's associated with your Apple ID, as the recipient. If you're going to be somewhere where you won't have cellular access (such as a foreign country) but only Wi-Fi, use your Apple ID-associated email address as the recipient to ensure you get it.

Each message will appear twice - once as being sent and once as being received - but other than that minor inconvenience, it is easy to message between your own devices.