You will need to render the character to a file or to an array, and then insert that array into the image.
Up to and including R2014a (but not later), you can work like this. Caution: this uses some magic smoke to do the job!
%use the decimal unicode character number, but only a subset is supported
%the use of HTML is documented magic... documented somewhere or other
%but you have to be aware it does not work for edit or text uicontrols
h = uicontrol('style', 'push', 'String', '<HTML>α');
%now shrink the size to just what is needed to contain the letter
%there is some slinky stuff about Extent working differently on different operating systems
%so you might need to fiddle a bit.
set(h, 'Position', get(h, 'Extent'));
%now use twice-magic incantations to render the uicontrol to an array
alpha_img = getPrintImage(handle(h));
Now you can store alpha_img as needed into the image you are building up.
If you do this for a number of different characters, you can re-use the same uicontrol and set its String differently. You should expect that the Extent will vary from character to character if you are using a proportional font.
