How to export a single schema from Oracle EE to Oracle XE?
There is no such one command to do this kind of migration of a schema because as you mentioned, Oracle Express edition doesn't support all the features that are used in the Enterprise edition. However, I tried to collect some of the useful parameters which can be used during expdp/impdp to modify or include/exclude (un)wanted objects and features.


   partition declarations can be omitted.


In that case you can use PARTITION_OPTIONS parameter in impdp.

PARTITION_OPTIONS=[NONE | DEPARTITION | MERGE].

PARTITION_OPTIONS


   storage declarations omitted or simplified (e.g. as little as possible initial space reserved for tables).


TRANSFORM parameter in impdp enables you to alter object creation DDL for objects being imported.

TRANSFORM = transform_name:value[:object_type]

TRNASFORM


   tablespace declarations omitted or replaced (a single tablespace used in XE).


You can use REMAP_TABLESPACE parameter during impdp on Oracle XE instance.

REMAP_TABLESPACE=source_tablespace:target_tablespace

REMAP_TABLESPACE


  materialized views with some advanced features can be simplified or replaced with simple views.


I think you need to import DDL of such materialized views into a SQL FILE then create view manually. 


  jobs can be omitted.


You can use EXCLUDE parameter to exclude jobs but currently, Data Pump Export/Import can exclude/include jobs which were created with DBMS_JOB.

Reference


  java functions (scarsely used but present in one pl/sql package) can be disabled or imported with compilation errors.


You can use EXCLUDE to ignore such java classes.

EXCLUDE=JAVA_CLASS

Now you can combine these parameters to form your expdp/impdp command.

Note: All included reference links are related to Oracle 11g version, please refer to the Oracle documentation related to your version.