You can load inventory quantity and value for materials in S/4HANA 2020 using IDoc Type MBGMCR04.
Yes, SAP S/4HANA Migration Cockpit is the “recommended by SAP” tool for loading data in S/4HANA. But there are exceptions to its efficacy. For example, IDoc type MBGMCR is exactly how it’s done in SAP Rapid Data Migration with SAP Data Services (On Premise), Migration Inventory Balances (W38).
If you’re not using SAP Data Services then Legacy System Migration Workbench (LSMW) is also relevant, using exactly the same IDoc approach that SAP delivers with Rapid Data Migration. What’s more, LSMW (more precisely, IDocs) remains essential for data migration in SAP Retail. For confirmation of necessity, see SAP Note 2538170 – Migration of Retail Objects to SAP S/4HANA on-premise.
MDA Workbench is a free and simple tool for creating such LSMW projects, and for creating a comprehensive kit of deliverables for data migration.