You'd need to use eval to first get the current values of the variable into a new variable, say existingVar. Then make up your new data and append it then use assignin to send the new variable back into the base workspace:
% Create data.
v1 = [1,2,3];
% Put v into the base workspace.
assignin('base','v1', v1)
% Now make some new variable v2
v2 = [4,5,6];
% Retrieve existing v1
v1 = evalin('base', 'v1')
% Append v2 to v1 and put results in v1.
v1 = [v1, v2]
% Send it back to the base workspace.
assignin('base','v1', v1)
% Verify: double check that v1 is what we expect.
v1 = evalin('base', 'v1')
