Look at the Alaska award charts here. If you enter Asia to Europe you will see you have 3 options. CX, SQ & AY.

BA is not an option and CX is from HKG only. One additional piece of crucial information is that AS awards only allow one partner with or without AS flights so you can't mix partners on one award.

From HND you could book seperate awards on CX. One to HKG and one to Europe. Since CX doesn't fly to TXL you would need to fly somewhere close and purchase a revenue ticket from the CX European gateway to TXL.

SQ is similar. You can book from North Asia to Europe via SIN as one award (if you have a huge stach of MP miles) but SQ also doesn't fly to TXL. You would need to fly to a close by SQ European gateway and find your own way to TXL. Scoot does fly non-stop from SIN to TXL but can't be booked with AS.

AY is your only true option as you can book, as one award, NRT-HEL-TXL if you can find space.

James