Why it is important to store your secret key securely (eg. on paper, multiple locations). XUMM doesn't allow you to export once imported.
XUMM lets users easily import or create an XRP account, but please keep your secret safe, offline, written down!
Users won't be able to discover their key after it has been safely stored on your mobile phone in XUMM. You have to write them down on a piece of paper.
Security is the reason for XUMM not allowing you to view/export your secret once imported.
People who want to steal your beloved XRP wont get access to your funds if they gain physical access to your smartphone, or (if your device is compromised), one could obtain this key by capturing your on-screen information. (XUMM also prevents making screenshots).
Store your secret somewhere safe
Store your secret written down on a piece of paper and also make a backup of that key on another piece of paper, stored elsewhere.
A digital copy is not advisable due to the easy accessibility if your account or computer gets compromised.