Can I have subledger accounting entries in Adjustment Only ledgers?

No, you can use Adjustment Only ledgers for General Ledger manual journals only.

However, you can define a Subledger level secondary ledger with valuation method enabled and use it for subledger journal adjustments. You can then create a ledger set to combine the secondary ledger with the primary ledger. Use the ledger set while running reports to obtain a complete accounting representation that includes both transactional data and adjustments.

You can also use the Accounting Event Exclusion with an Adjustment-Only Accounting Method Representation feature to model adjustment-only scenarios. Configure the Exclude option for specific journal entry rule set assignments in the accounting method so that selected accounting events are excluded from one accounting representation while still being accounted in another.

For more information, see Accounting Event Exclusion for Adjustment-Only Accounting Method Representation.