Please see the following example code.
The basic idea is to load the other project and then set the design constraint. Note, that the inputs in the constraint you are importing must match the input order of the design inputs. If the order does not match, you can use mbcdoe.designconstraint.MatchInputs.
Hope that helps,
Create Space-Filling Design
CreateDesign defaults to creating a design for the outer (global) level.
sfDesign = CreateDesign(TP, ...
'Type', 'Latin Hypercube Sampling',...
'Name', 'Space Filling');
Add Boundary Constraints
Load boundary constraints from another project file and add to design.
otherProject = mbcmodel.LoadProject( [matlabroot,'\toolbox\mbc\mbctraining\Gasoline_project.mat']);
boundaryConstraints = otherProject.Testplans(1).BoundaryModel('global');
% Design constraints are specified as an array of
% mbcdoe.designconstraint objects.
sfDesign.Constraints = boundaryConstraints;