External Table Means
1. Data is already present , with certain properties like compression , partition , every record in file has certain way to identify and interpret columns etc.
2. When you map Hive to the external table it should have the same attributes of what data has when it was stored in HDFS.
3. If you had enabled compression on Hbase then hive will automatically detect.
No, you can't specify any encoding scheme and compression for HBase table from hive, you need to do it from the HBase shell.
There are limited encoding schemes which are applicable for HFile(FASTDIFF,DIFF,PREFIX) and doesn't include Avro but you can store Avro data in HBase column and define the schema for that data by using Hive.