Hi,

Yes it is possible to assign a leads of one salesperson to another salesperson by setting the particular rule to corresponding salesperson by which the salesperson will also get the same lead.
for example if salesperson A receive a leads from Belgium and also wanted the salesperson B to receive the same set of leads, in that case you can set rules in such a way that the salesperson B also receives the leads from Belgium.

Hope it helps

In case if you have massive leads to modify , you need to add new action : Change assignment , It is just like merge action but only change saleperosn or may be team it self.

Meanwhile as a workaroud  use export and import feature.

Option1)

The best way is to Export your leads/Opportunity with ExternalID for that Salesperson.

Replace the Salesperson in XLS/CSV file and import it back.

Option2) 

Write a server action with python code.

Hello,

Open the Lead/Opportunity form view and change the salesperson.