The smallest value allowed for the newTimeStep input argument describe in the documentation for retime is 'secondly', but you can create a vector of new times with whatever spacing you want and use that as the newTimes input argument for retime.
% Get a starting time
N = datetime('now');
N.Format = 'HH:mm:ss.SSSS';
% Make the timetable spanning 1 second with some arbitrary data to retime
tt = timetable(N+seconds([0; 1; 3; 6; 10]./10), [0; 1; 10; 100; 1000])
% New time vector with spacing (timing?) of 0.1 second
tv = (N:seconds(0.1):N+seconds(1)).'
% retime it
newtt = retime(tt, tv, 'linear')
The value of Var1 for the third entry in tv, N+seconds(0.2), should be 5.5 which is halfway between the value of 1 for N+seconds(0.1) and the value of 10 for N+seconds(0.3).
