No TBLAST option available - how to work with Balance Data?

Summary: Number of policies is a balance account. There is no option to specify an account to be Time Balance Last. How to resolve this best?

Content (required):

We want to load the number of policies into our application. This data will be loaded monthly. The normal aggregation in the period dimension is from the months into the quarters and from there into the year total.

The number of policies is a balance type. When loading them into January, February and March, the Quarter1 should show the March data, and not the sum of these.

How to accomplish this, without loading a begin balance the delta of the policies every month?