Enter the weaning information as normal but leave the weight column blank. When done, highlight all of the weight columns by dragging vertically down with your mouse button (as shown below) and then right-click and choose set cells. Enter the weight you want to appear in each row and click OK. This can be used in any data input screen if required.

An alternate way of doing this is to enter the date for the weanings you want to enter a weight against in the green filter line as shown below (green arrow) and then click on the "Enter weight" icon at the top right of the screen (shown by red arrow):

The box shown will pop up and you can now enter the total weight for the pigs weaned on this day and click OK and it will allocate this to the sows weaned. 

If you are unable to enter a weaning weight, please check the setting in General menu, Setup, Validation tab, Sow data tab as shown below and change the option to None if applicable: