There is a way to set an ID value, but there are risks involved, since you need to query the database directly. Yes yes, SharePoint geeks and cracks will go NO NO NO, but since you're not changing the DB schema you're all good. Let me explain why it's safe:
- Microsoft doesn't recommend querying the Content database directly since it might void the warranty. This might happen if you go berserk with UPDATE TABLE and DROP VIEW SQL queries. While you're not changing the schema, you're fine. Microsoft tools that check if the warranty is void check database schemas and not the content.
- All content migration tools from Metalogix, Dell, AvePoint etc. allow you to tick a box called "Preserve Item IDs". There is no magic involved - they all query the Content database and set NextAvailableId. If that would void the warranty, most of the migrated SharePoint farms would end up not being supported by Microsoft? I hardly doubt it :)
- This is not a valid point, but still - sometimes you just can't get over a problem without messing up with the Content database a bit. See this.
Anyway, if you decide to go for it:
In your workflow code you need to do some SQL magic before copying the item to your new list:
UPDATE ContentDB.dbo.AllListsAux set NextAvailableId='1' where ListID='GUID'
This way you can set the ID of the next item that will be added to the list. This could be a solution if the items are not added to the list in any other way except by that single workflow. If not, you'll end up having a small issue. Eg.: you have items with IDs 1, 3, 4 and 5. You set NextAvailableId to 2 and insert an item. You end up having items 1, 2, 3, 4 and 5, but NextAvailableId is incremented by one (which means it would currently be set to 3). If you add an item and click Save, the item wouldn't be saved but the NextAvailableId would still be incremented by one (leaving it at for). So you would need to click Save button 2 more times so the NextAvailableId is set to 6. Only then you will be able to save the item.
Otherwise just go for a custom number column and assign old ID to the new column.
Hope this helps!