Unfortunately it isn't this easy. Several *A carriers are stingy with codeshares even though flight is operated by another *A carrier. For example from SAS Sales Info:

Flights not valid for accrual and redemption
Accrual and redemption is not permitted on the following flight number ranges:

LX Code Share Operating Partner
LX 3240 – 3499 operated by UA
LX 3550 – 3599 operated by OS
LX 3600 – 4079 operated by LH
LX 4240 – 4249 operated by JP
LX 4250 – 4259 operated by OU
LX 4300 – 4309 operated by TG
LX 4580 – 4599 operated by TP
LX 4640 – 4659 operated by AC
LX 4660 – 4749 operated by SK
LX 4750 – 4789 operated by JK
LX 4790 – 4799 operated by KF
LX 7400 – 7449 LX rail service 'Fly Rail'


Flight number ranges excluded for accrual and redemption
The following flight numbers ranges are excluded for accrual and redemption.
Note: The flight number ranges may be changed.

Flight number range Operated by
JK 1000-4999 Charter flights
JK 7500-7999 Codeshare with other carriers
JK 8801-8999 Lufthansa
JK 9000-9099 SAS
JK 9200-9250 Austrian Airlines
JK 9251-9299 Cubana de Aviacion
JK 9300-9399 Lufthansa
JK 9400-9499 Air Littoral
JK 9500-9550 Portugalia
JK 9551-9599 Air Canada
JK 9600-9699 bmi british midland
JK 9700-9750 Varig


I don't know how hard they enforce these rules. And if you fly with SK you probably get a BP just telling the SK flight number which should be eligble using RetroRegistration. Haven't tried it myself though.