Project project = new Project(dataDir + "Blank2010.mpp");

// Add new task and resource
Task task1 = project.getRootTask().getChildren().add("Task");
Resource rsc1 = project.getResources().add("Rsc");

// Assign the resource desired task
ResourceAssignment assn = project.getResourceAssignments().add(task1, rsc1);

ResourceAssignment assignment = project.getResourceAssignments().toList().get(0);

// Custom attributes which is visible in "Resource Usage" view can be created
// with ExtendedAttributeDefinition.CreateResourceDefinition method.
ExtendedAttributeDefinition resCostAttr = ExtendedAttributeDefinition
        .createResourceDefinition(CustomFieldType.Cost, ExtendedAttributeResource.Cost5, "My cost");

project.getExtendedAttributes().add(resCostAttr);

ExtendedAttribute value = resCostAttr.createExtendedAttribute();
value.setNumericValue(BigDecimal.valueOf(1500));
assignment.getExtendedAttributes().add(value);

// Custom attributes which is visible in "Task Usage" view can be created with
// ExtendedAttributeDefinition.CreateTaskDefinition method
ExtendedAttributeDefinition resCostAttr2 = ExtendedAttributeDefinition
        .createTaskDefinition(CustomFieldType.Cost, ExtendedAttributeTask.Cost5, "My cost for task");

project.getExtendedAttributes().add(resCostAttr2);

value = resCostAttr2.createExtendedAttribute();
value.setNumericValue(BigDecimal.valueOf(2300));

assignment.getExtendedAttributes().add(value);

project.save(dataDir + "AddExtendedAttributesToResourceAssignment_out.mpp", SaveFileFormat.MPP);