The characters in such a monospace font are ordered in increasing ASCII codepoint row-major order.
This means that you can derive the cell offset by simple arithmetic. Given the codepoint base of the top left character, the cell index col,row of a codepoint ch is:
col= (ch-base) % columns
row= (ch-base) / columns
You can then compute X and Y of the rectangle by multiplying the cell index by the width/height of a cell.
Given the computed extents of each character, you can then emit a FNT file based on the FNT specificationFNT specification, or try to find a FNT generator that you can provide with input regions and a bitmap.