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 exp_blocks_block
. blockdefinition_id
is what you looked in step #2. site_id
and 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 exp_blocks_atom
. 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 exp_channel_data
table.
If you run into trouble, you can fill out Blocks' support form and I'll be happy to help.