A fine-writing nib can be broadened by grinding only within narrow limits, because the material to do it with is not there: line width comes from the iridium tip at the point of the nib, and material once removed cannot be put back. Regrinding alone will therefore not turn an F or EF nib into a B; at best half a width grade can be gained, along with a smoother, less scratchy feel.
The proper route leads through a nib workshop: specialists can fit a new, larger iridium tip and then grind the nib to the width required. That is the only way to achieve a substantial increase in width without replacing the nib itself. Alternatively a suitable replacement nib can be sourced; specialist pen dealers and spare-parts suppliers carry individual Montblanc nibs, though there is no organised exchange for nibs. Whichever route is taken, bear in mind that a broader nib lays down more ink: on absorbent paper the writing may feather or bleed through as a result.