Code samples used to help explain the steps requried for Sitecore Experience Anayltics custom dimensions.

CustomerTypeVisitDimensionBase.cs

using Sitecore.Analytics.Aggregation.Data.Model;
using Sitecore.ExperienceAnalytics.Aggregation.Dimensions;
using System;

namespace TheCodeAttic.ExperienceAnalytics.Aggregation
{
    public class CustomerTypeVisitDimensionBase : VisitDimensionBase
    {
        public CustomerTypeVisitDimensionBase(Guid dimensionId): base(dimensionId)
        {
        }
       
        public override string GetKey(IVisitAggregationContext context)
        {
            //Making a call to our CRM system to determine the contact type. 
            //In a fuller implementation, we could track the customer type as a custom contact facet.
            return CRMInterface.CustomerType(context.Visit.ContactId);
        }

        public override bool HasDimensionKey(IVisitAggregationContext context)
        {
            //Need to confirm that key will / does exist.
            return Guid.Empty != context.Visit.ContactId;
        }
    }
}

MetricCalculationsForVisitBase.cs

///Extracted from Sitecore.ExperienceAnalytics.Aggregation.Dimensions.DimensionBase
namespace Sitecore.ExperienceAnalytics.Aggregation.Dimensions
{
    public  class MetricCalculationsForVisitBase
    {       
        public virtual SegmentMetricsValue CalculateCommonMetrics(IVisitAggregationContext context, int count = 0)
        {
            Assert.ArgumentNotNull((object)context, "context");
            if (context.Visit.Pages == null)
                return (SegmentMetricsValue)null;
            return new SegmentMetricsValue()
            {
                Visits = 1,
                Value = context.Visit.Value,
                Bounces = context.Visit.Pages.Count == 1 ? 1 : 0,
                Conversions = Enumerable.Count<PageEventData>(Enumerable.SelectMany<PageData, PageEventData>((IEnumerable<PageData>)context.Visit.Pages, (Func<PageData, IEnumerable<PageEventData>>)(page => (IEnumerable<PageEventData>)page.PageEvents)), (Func<PageEventData, bool>)(evt => evt.IsGoal)),
                TimeOnSite = Enumerable.Sum<PageData>((IEnumerable<PageData>)context.Visit.Pages, (Func<PageData, int>)(page => DimensionBase.ConvertDuration(page.Duration))),
                Pageviews = context.Visit.Pages.Count,
                Count = count
            };
        }
    }
}

TheCodeAttic.ExperienceAnalytics.xml

<?xml version="1.0"?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <experienceAnalytics>
      <!--FROM <web root>\App_Config\Include\ExperienceAnalytics\Sitecore.ExperienceAnalytics.Reduce.config-->
      <reduce>
        <dimensions>
          <dimension id="{DBBB04C0-84DC-41F2-9B06-C2207284923A}" type="TheCodeAttic.ExperienceAnalytics.Aggregation.CustomerTypeVisitDimensionBase, TheCodeAttic.ExperienceAnalytics.Aggregation" />
        </dimensions>
      </reduce>
      <!--FROM <web root>\App_Config\Include\ExperienceAnalytics\Sitecore.ExperienceAnalytics.Aggregation.config-->
      <aggregation>
        <dimensions>
          <dimension id="{DBBB04C0-84DC-41F2-9B06-C2207284923A}" type="TheCodeAttic.ExperienceAnalytics.Aggregation.CustomerTypeVisitDimensionBase, TheCodeAttic.ExperienceAnalytics.Aggregation" />
        </dimensions>
      </aggregation>
    </experienceAnalytics>
  </sitecore>
</configuration>
<!--Be sure to rename from  'XML' to 'config' if you plan to use-->