how do i add a new empty column sequentially?

Let’s filter the numeric columns then using get_loc obtain the location in the dataframe correspoding to the start and end location of the numeric columns, finally use reindex with fill_value=0 to reindex accordingly:

c = dd.filter(regex=r'^\d+$').columns
l1, l2 = dd.columns.get_loc(c[0]), dd.columns.get_loc(c[-1])
idx = np.hstack([dd.columns[:l1], np.r_[c.astype(int).min():c.astype(int).max() + 1].astype(str), dd.columns[l2 + 1:]])
dd = dd.reindex(idx, axis=1, fill_value=0)

   a  1  2  3  4  5  6  b
0  1  1  1  0  1  0  1  1