Use Supporting References in SLA and you can see SLA generated and manual JEs in the same inquiry.

Setup
First you have to define a supporting reference eg PO Number, then define which Event Classes and the source that has the PO Number, whoohooo this even goes across appl_ids so following PO example, assign it to Invoices in AP and Purchase Orders in PO.

Supporting References can be assigned to line (JLD) or header level (AAD) for Accounting Engine Generated Journals and directly entered for Manual JEs from the page.

In the JLD, you assign to the JLT, which is a bit tedious though needed if the value can be different for different JLTs in the same event class. In the AAD, choose header assignments

NOTE: If you have maintain balances, you can only assign to the line level.

Entering
For journals generated by the accounting engine your JLD/AAD setups will automatically populate, for manual SLA journals you have to remember to enter them in the UX.

Viewing
In the standard inquiry for journal header/line you will be able to see the supporting reference as a column that can be used as a query criteria.  There are no standard reports but the data model is fairly simple. There are analgous header and lines for supporting references as well as a balances table.