http://tech.blog.aerie.jp/entry/2015/09/25/103325

ExpressionCombiner.cs

public static class ExpressionCombiner
{
    public static Expression<Func<T, bool>> OrElse<T>(
        params Expression<Func<T, bool>>[] expressions)
    {
        return OrElse(expressions.AsEnumerable());
    }

    public static Expression<Func<T, bool>> OrElse<T>(
        this IEnumerable<Expression<Func<T, bool>>> expressions)
    {
        if (!expressions.Any())
        {
            throw new ArgumentException($"parameter [{nameof(expressions)}] is empty.", nameof(expressions));
        }

        var lambda = expressions.First();
        var body = lambda.Body;
        var parameter = lambda.Parameters[0];

        foreach (var expression in expressions.Skip(1))
        {
            var visitor = new ParameterReplaceVisitor(expression.Parameters[0], parameter);
            body = Expression.OrElse(body, visitor.Visit(expression.Body));
        }

        lambda = Expression.Lambda<Func<T, bool>>(body, parameter);
        return lambda;
    }
}

ParameterReplaceVisitor.cs

internal class ParameterReplaceVisitor :
    ExpressionVisitor
{
    private readonly ParameterExpression _originalParameter;
    private readonly ParameterExpression _newParameter;

    public ParameterReplaceVisitor(
        ParameterExpression originalParameter,
        ParameterExpression newParameter)
    {
        this._originalParameter = originalParameter;
        this._newParameter = newParameter;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node == this._originalParameter)
        {
            return this._newParameter;
        }

        return base.VisitParameter(node);
    }
}