I'm looking to do the following query:

PHP Code:

INSERT INTO users ( field1, field2, field3 )  (SELECT field1, field2, field3 FROM tempusers WHERE email='some@email.com') 


however, I also want to put a value in one field as set manually. I've tried many different ways and they do not work. This is one thing I've tried, which illustrates what I'm trying to accomplish:

PHP Code:

INSERT INTO users ( field1, field2, field3, field4 )  (SELECT field1, field2, field3 FROM tempusers WHERE email='some@email.com'), 'field4data' 


but that did not work. I know you can use SELECT statements on the fly to retrieve data to insert into fields but can you do that alongside directly provided values as well in the same query, or is that something MySQL can't do?

Of course you can. the second query is almost correct.
It should be

INSERT INTO users ( field1, field2, field3, field4 )  
SELECT field1, field2, field3,'field4data'
FROM tempusers WHERE email='some@email.com'

In the select, you can "add columns" by simply adding the value in the select list, that's what 'field4data' is.

And if you want to do that for several rows, you can use the UNION keyword

INSERT INTO users ( field1, field2, field3, field4 )  
SELECT field1, field2, field3,'field4data' FROM tempusers WHERE email='some@email.com'
UNION
SELECT field1, field2, field3,'field4data' FROM tempusers WHERE email='one@email.com'
UNION
SELECT field1, field2, field3,'field4data' FROM tempusers WHERE email='else@email.com'