>> dsec=seconds(str2double(extractBefore(t,'.'))) + ...
This is the most straightforward workaround I can think of within the limittions of the (somewhat hampered) durations class which has limited input options for formatting input and the helper functions such as seconds that are only base numeric classes aware.
The datetime class is still pretty young; it has much left to be worked out in order to make it fully functional over niche usage such as yours.
You'll probably have to build a wrapper class of functions to hide all the internal machinations required; the above just illustrates that if you can break the whole number of seconds precision required into pieces that are each within the precision of a double that the duration object itself can deal with them to that precision -- at least storing an input number. I've not tested about rounding when try to do arithmetic with the result.