There is no officially sanctioned way to do this for Blocks. And, as far as I'm aware, there is no third-party add-on that will make this easy.
Like sheferd said, you're going to want to make a backup, find the field ids in the database, etc.
In Blocks, create a new block type by going to Add-Ons → Modules → Blocks. Create a block type with a single Wygwam field in it.
This block definition will be stored in the database in
exp_blocks_blockdefinition, and its Wygwam field will be in
exp_blocks_atomdefinition. You'll want to grab both of those IDs.
Here's where you might want to drop into a custom PHP script, though it might be possible to do with just SQL.
Create a block by adding to
blockdefinition_id is what you looked in step #2.
entry_id mean what you think they mean.
field_id is the ExpressionEngine field that this Block will belong to (retrieved from Admin → Channel Administration → Channel Fields).
order will probably be 1, which means it is the first block for that field. When you insert into this table, take note of the ID that is autocreated; you'll need it for when you add your content as an atom.
Create an atom by adding to
block_id is the ID you got when you created a row in step #3.
atomdefinition_id is what you looked up in step #2.
data is your Wygwam data, retrieved from the
If you run into trouble, you can fill out Blocks' support form and I'll be happy to help.