I don't see any way to do so, directly, no.
AFAICT, none of the ways to specify the model allow you to enter a constant for a given term, nor can you build a LinearModel object other than through fitlm for an OLS fit; you can't try subterfuge of fitting the ordinary model and then replacing the coefficients, either; they're read-only.
The only way I see would be by the other subterfuge of creating a set of data that will produce the desired coefficents and fitting that. That would be relatively easy to have code to do since the intercept is easy-enough to calculate, one can generate a set of points that fit the proposed line and fit those. However, of course, the statistics other than the coefficients themselves will be bogus because the fit will be exact.
So the net result for which reason you propose wanting to do so will not produce the desired results; you'd still have to compute those intervals, etc., directly.
Of course, when one sets the value of the slope, none of the assumptions for those statistics are valid any longer, anyways...
