No.
At the end of a "syms" you can add the keyword "real" to add the assumption of real-valued to each variable mentioned. You can also add "positive" to add the assumption of positive to each variable mentioned. However, there is no corresponding flag to syms for "nonnegative", so you will need to assume() for that.
You could write a small function
function NNsyms(varargin)
for K = 1 : nargin
evalin('caller', sprintf('syms %s; assume(%s >= 0)', varargin{K}, varargin{K});
end
and then use 'NNsyms' everywhere you would otherwise use syms where you wanted the nonnegative assumption.
I poked into the code and found that you would have to modify a couple of Mathworks-provided routines to support this assumption by way of a key word. It would not be bad, though. Expand some cell arrays used for checking, modify setToMuPADSet.m slightly to add a mapping to
'Dom::Interval([0], infinity)'
where the [] around the 0 is important.
