Hey Andreas,
I would assume so according to the knowledgebase article I have linked:
"
- If a certain tool or feature does not sync to a standard sandbox account, any properties that are dependent on that feature will also not sync. For example, since subscription types do not copy to standard sandbox accounts, all contact properties based on subscription types (e.g., Opted out of email) are also not copied. In this scenario, you’d need to manually recreate subscription types in the sandbox account to match your main account. Additionally, scoring ruleset syncing is not supported in your sandbox account."
But I’m not entirely sure to be honest, you may want to test this out prior to doing your main sync.
It looks like from the article that yes this would be just pulling data from your main CRM, not overwriting existing sandbox data, but you may again want to test this with a dummy account or do a trial run first.