Indeed, GLCD Font Creator allows to export fonts in fixed-width format only if font height does not exceed 8 pixels. Unfortunately, my program, uGLCDfonts, follows what GLCD Font Creator produced.
There are two solutions. One which I, in my laziness

, use - simply changing the first column where necessary (mostly digits and space for ease of use when displaying numbers) to constant value in variable-width font table produced by GLCD Font Creator - before applying uGLCDfonts. Obtained font is, naturally, of variable-width type but with chosen fonts or all fonts of equal width.
The second solution requires removal of first column in variable-width font table produced by GLCD Font Creator (exported for GLCD, not for new library) and change of the sentence
//WARNING: This Font Require X-GLCD Lib.
in font file description to
//WARNING: This Font is usable only with MikroE GLCD Lib.
as uGLCDfonts uses it to differentiate between fixed- and variable-width font files.
I will think of a simpler solution where uGLCDfonts program would have an option to produce fixed-width font table from variable-width one obtained from GLCD Font Creator.