If you have preset the option in System Preferences > Keyboard > Keyboard Shortcuts for the Tab key to move keyboard focus between "All controls" there is a way.
Press Fn-Control-F2 and the icon for the Apple menu should be highlighted. (You can move to other menus by using the right and left arrow keys if needed, though not in this case).
Press the down arrow key. This should open the Apple menu.
Press the down arrow key again until the item System Preferences is highlighted.
Press the Return key, and System Preferences should open.
Press the tab key, and the items in the System Preferences array should be highlighted in turn. Once one of the is highlighted, you can also use the arrow keys to move the selection choice.
Move the selection choice until the item Universal Access is highlighted.
Press Option-Spacebar which should open that item.
Use the Tab key to highlight the tab Mouse.
Press Option-Spacebar to open that tab.
Use the Tab key to highlight the "Press the Option key five times to toggle Mouse Keys" item.
Press Option-Spacebar to checkmark that item.
Then, when you need to use Mouse Keys you can press Option five times in a row and it will become active; press it five times in a row again to toggle Mouse Keys off. Note - while Mouse Keys is active, some normal keyboard functioning (such as being able to type characters) may not be available.