inserting into a db, from multiple tables

[afrika]

hello,
I have an insert SP as below

insert into accounting (item, price, quantity, purchaseID, username)
select item1, price1, quantity1 from Accounting where userID = @user

however i would like to combine 2 or more table to insert into the accounting table above to augument for the purchaseID and username.

e.g. something like this;

insert into dbo.accounting (item, price, quantity, purchaseID, username)
select item1, price1, quantity1 from dbo.Accounting where userID = @user , dbo.users purchaseID, username INNER JOIN     dbo.Users ON dbo.accounting.username = dbo.Users.username

how do i go about this correctly ?

thanks
Afrika