Add query hints to a CROSS APPLY
You can't, as far as I'm aware.
One might try things like:
OPTION (TABLE HINT (sp, READUNCOMMITTED))

...but this will fail for things that aren't real tables.

Table-valued or OPENROWSET function 'sp' cannot be specified in the TABLE HINT clause.

It would also fail because the desired hint is semantic-affecting.
If the thing you need to access doesn't respond to setting the isolation level, you'll have to report the deficiency and hope for a fix.
There was a Connect item about DMVs and whatnot not respecting isolation level, and as best I recall, the official word was that isolation level was not guaranteed for system functions.
Aaron Bertrand said:

There are some built-ins that simply don't obey isolation level no matter how the hint gets there (e.g. OBJECT_SCHEMA_NAME().


Also, as Martin Smith remarked:

The issue is really how to apply it to a TVF - not a CROSS APPLY
SELECT * FROM sys.dm_db_stats_properties(1,1) WITH (NOLOCK)

also fails. But conversely this succeeds:
SELECT *
FROM sys.objects obj WITH (NOLOCK) 
INNER JOIN sys.stats stat WITH (NOLOCK) ON stat.object_id = obj.object_id  
CROSS APPLY sys.objects obj2 WITH (NOLOCK)