creating a pool of items

Hello,
I'm a university student in Psychology. I have French as my second language. During my study in cognitive psychology, I’m doing a study on « divided visual attention for children » I have been using OpenSesmae since, September 2014. Everything starts well till a problem occurred. In fact, it is about a matrix containing numbers (targets) and letters (distractors) (see the picture below). The assignment was to drive the attention to the four gaps in the matrix in order to divide the attention into 4 locations (I’m in control of this variable) in order to get a right result (which number is shown in the required location). Every trial contains 8 matrixes and the subject is due to give a right response after every trial. I have constructed 400 matrixes. Here appears the problem: I did not find how to create a block (pool/ reservoir) for all the trials because it will be exhausting to construct 400 skeletons as mentioned in the software picture. My question is: how can I create a block for each trial containing the 8 matrix, so that the software can select the required matrix everytime? thank you

http://img.cogsci.nl/upload.php
http://img.cogsci.nl/?q=5485e7c77f763